Research @ KICP
June 1, 2005
Modeling Formation of Galaxy Clusters
by Daisuke Nagai
a recent paper, Nagai and Kravtsov showed that a process called "tidal stripping" operates with different efficiency on dark matter and stars associated with cluster galaxies. When the galaxies and their dark matter accrete onto the cluster, the dark matter, owing to its more extended distribution, is stripped more efficiently than the stars that are located near the center of the galaxy and are tightly bound by gravity. Consequently, the radial distribution of galaxies depends on whether the galaxies are selected using their dark matter or stellar mass. Selection based on the dark matter mass favors galaxies less affected by tidal mass, located preferentially on the outskirts of clusters. This results in a more extended, "flatter" radial distribution. Selection based on the stellar mass does not suffer from such "selection bias" and results in a more centrally concentrated distribution. This difference in selection is important when one compares simulations to the data. The figure shows that the surface density of galaxies in projected radial bins does not agree with the optical/IR observations of cluster galaxies (triangles and solid circles), if the simulated galaxies are selected using their total mass (blue dashed line). The radial distribution of galaxies selected using stellar masses (red dotted line), on the other hand, is in good agreement with the data. The stellar mass selection is similar to the selection using the K-band luminosity of galaxies used to construct the observed radial profile. It is therefore not too surprising that stellar mass selection works better. Nevertheless, this example illustrates the importance of star formation in realistic cluster simulations.
Sunyaev-Zeldovich Effect (SZE) signal and cluster gas mass. The numerical simulations performed by Daisuke Nagai are used to study the effects of galaxy formation on the SZ scaling relations, and these theoretical predictions are tested using SZE+Chandra cluster observations for a sample of 36 clusters obtained by a group led by the KICP faculty John Carlstrom. These comparisons show that simulations neglecting galaxy formation (red triangles and dotted line) are inconsistent with the observed correlation. The simulations that include gas cooling and star formation (blue triangles and dashed line), on the other hand, are in good agreement with the data. This comparison highlights the importance of inclusion of galaxy formation in cluster simulations and shows that the current generation of simulations produce clusters with gross properties remarkably similar to their observed counterparts.